Surface Acoustic Wave (SAW) devices are semiconductor devices that use surface acoustic waves whose energy is transmitted convergently on the surface of a solid. In general, a SAW device includes a layer of a piezoelectric material and one or more interdigitated transducer (IDT) electrodes formed on the piezoelectric layer. The surface acoustic wave may be excited by applying an electrical signal to an IDT electrode. Electrical signals are correspondingly generated across the opposite IDT electrode as surface acoustic waves pass the electrode. Typical piezoelectric materials include bulk monocrystals of quartz, as well as layers of LiNbO.sub.3, LiTaO.sub.3, AlN, or ZnO grown on a substrate.
In general, the active frequency (f) of a surface acoustic wave device is determined by the formula f=v/.lambda., where .lambda. is the wavelength and v is the propagation velocity of the surface acoustic wave in the piezoelectric material. The wavelength .lambda. is dependent on the spacing frequency of the interdigitated electrodes and the crystal orientation of the surface of the material through which the wave passes. Typical propagation velocities v for exemplary materials are as follows: 3500 m/sec to 4000 m/sec for a monocrystalline LiNbO.sub.3 layer, and 3300 m/sec to 3400 m/sec for a monocrystalline LiTaO.sub.3 layer. The propagation velocity v is relatively high at approximately 3000 m/sec for a ZnO film on a glass substrate.
The active frequency f can be increased either by increasing the propagation velocity v or by decreasing the wavelength .lambda.. Unfortunately, the propagation velocity is restricted by the material properties of the piezoelectric layer. The wavelength .lambda., which is determined by the width, spacing, and arrangement of the IDT electrodes, is limited by the lower limits of existing processing technologies. In a typical interdigitated electrode having an array of alternating equally spaced electrode fingers with a common width w and a common spacing s, for example, the wavelength is determined by the formula .lambda.=2s+2w. Other electrode arrangements will have other relationships between the wavelength, electrode width, and electrode spacing.
Submicron geometries may be difficult to fabricate using conventional materials, and long term reliability is limited by metal migration effects. For example, many existing optical lithography technologies cannot be used to fabricate a line/groove structure having a width of less than 0.8 .mu.m. In addition, a narrower line width lowers the fabrication yield. For these reasons, the maximum frequency of many existing SAW devices in practical use is approximately 900 MHz.
A surface acoustic wave device having interdigitated electrodes on a LiNbO.sub.3 substrate may have a surface acoustic wave velocity of 4003.6 m/s, a coupling coefficient of 5.57%, and a frequency temperature coefficient of -72 ppm/K, for example. In a device having alternating equally spaced interdigitated electrodes with 1 .mu.m wide electrodes and 1 .mu.m spaces between electrodes, the frequency will be approximately 1 GHz. In order to achieve a 2.5 GHz device, the electrodes would need to have a width and spacing of approximately 0.4 .mu.m.
In the case of SAW devices including a piezoelectric film on a substrate, plural surface acoustic waves are excited if the sound velocity of the substrate is different than the surface acoustic wave velocity of the piezoelectric film. These surface acoustic waves are called zeroth mode waves, first mode waves, second mode waves, etc. according to the order of increasing velocity. The velocities of all modes depend on the substrate, as well as the piezoelectric film. The use of substrates having higher sound velocities results in higher velocities for all modes of the surface acoustic wave in the device. That is, the surface acoustic wave velocity increases in proportion to the sound velocity of the substrate.
A multilayer surface acoustic wave device is disclosed, for example, in a reference by Shiosaki et al. entitled High-Coupling and High-Velocity SAW Using ZnO and AlN Films on a Glass Substrate, and appearing in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. UFFC-33, No. 3, May 1986. The SAW device disclosed by the Shiosaki et al. reference includes a borosilicate glass sheet substrate, a C-axis-oriented AlN film on the substrate, and a C-axis-oriented polycrystalline ZnO film on the AlN film opposite the substrate. Aluminum IDT electrodes are included between the AlN and the ZnO films. With this structure, a maximum coupling coefficient of 4.37% was reportedly obtained where the phase velocity was 5840 m/s. The frequency temperature coefficient of this device was 21.0 ppm/.degree.C. at 25.degree. C. The phase velocity of this device, however, is still relatively low. Accordingly, high frequency performance is limited.
A surface acoustic wave device having a relatively higher propagation velocity is disclosed in U.S. Pat. No. 5,221,870 to Nakahata et al. The patent discloses a SAW device having a silicon semiconductor substrate, a diamond film on the substrate, a ZnO piezoelectric layer on the diamond layer, and interdigitated transducer electrodes on the piezoelectric layer. For the diamond film, both a single crystal and polycrystalline film are suitable. However, a monocrystalline film is more favorable, because there is less acoustic scattering in monocrystalline diamond as compared to polycrystalline diamond.
Diamond is a preferred material for many semiconductor devices because of its hardness, relatively large bandgap, high temperature performance, high thermal conductivity, and radiation resistance. Moreover, diamond is desirable for SAW devices because it has relatively large values of acoustic velocities. See, for example, "SAW Propagation Characteristics and Fabrication Technology of Piezoelectric Thin Film/Diamond Structure", by Yamanouchi et al., 1989 Ultrasonics Symposium, pp. 351-354, 1989. Moreover, combining diamond with relatively low velocity piezoelectric materials results in higher SAW velocities; thus, the demands on line spacing may be reduced for a given frequency of operation as disclosed, for example, in "High Frequency Bandpass Filter Using Polycrystalline Diamond", by Shikata et al., Diamond and Diamond Related Materials, 2 (1993), pp. 1197-1202.
U.S. Pat. No. 5,235,233 to Yamamoto and entitled Surface Acoustic Wave Device discloses a SAW device including diamond, an AlN layer on the diamond layer and IDT electrodes on the AlN layer. In another embodiment, an intervening layer of SiO.sub.2 is provided between the diamond and AlN layers. High electromechanical coupling coefficients and high phase velocities are reportedly provided by the devices.
To further monitor and control the temperature of a diamond SAW device, U.S. Pat. No. 5,235,236 to Nakahata et al. and also entitled Surface Acoustic Wave Device discloses a thermistor formed by a semiconducting diamond layer which, in turn, is supported on an insulating diamond layer of the SAW device. The thermistor cooperates with a heater to control the operating temperature of the SAW device.
U.S. Pat. No. 4,952,832 to Imai et al. entitled Surface Acoustic Wave Device also discloses a SAW device including polycrystalline or single crystal diamond that may be used as a filter, a resonator, a delay line or a signal processing device and a convolver. See U.S. Pat. Nos. 5,221,870 and 5,160,869 both to Nakahata et al.; High-Frequency Surface Acoustic Wave Filter Using ZnO/Diamond/Si Structure, by Nakahata et al., from the International Conference on the Applications of Diamond Films and Related Materials, pp. 361-364, (1993); and High-Frequency Surface Acoustic Wave Filter Using ZnO/Diamond/Si Structure, by Nakahata et al., 1992 Ultrasonics Symposium, pp. 377-380, (1992).
The device of the Nakahata et al. U.S. Pat. No. 5,221,870, for example, may have a relatively high surface acoustic wave velocity of more than 10,000 m/sec. Accordingly, the high surface acoustic wave velocity v reduces the necessity of fine IDT electrodes. In particular, Nakahata et al. discloses that IDT electrodes having a line width of 1 .mu.m and a spacing of 1 .mu.m may produce a surface acoustic wave with a frequency as high as 2 GHz. However, a polycrystalline diamond film may produce acoustic scattering and may require polishing. A single crystal diamond film may provide better performance, however, single crystal may be expensive and difficult to produce.